1. Technical Field
The present invention relates in general to electrical circuitry and in particular to a power supply for an electrical circuit. Still more particularly, the present invention relates to a multiple output power supply for an electrical circuit that has rapid short circuit protection.
2. Description of the Related Art
As illustrated in FIG. 1A, a conventional low cost direct current (DC) power supply 10 may have an alternating current (AC) input 8 and multiple DC outputs 11, 12, and 13. Because power supply 10 is a low cost power supply, not all of outputs 11-13 have a current limit. That is, outputs 11 and 12 may be limited to a maximum output current, while output 13 may not be so limited. In order to protect power supply 10 from damage in the event of a short circuit, outputs 11-13 of power supply 10 are connected to a short circuit protection circuit comprised of under-voltage detection circuit 14, time delay 16, and shutdown latch 18.
As shown, each of outputs 11-13 forms an input of under-voltage detection circuit 14, which transmits an under-voltage signal 15 to time delay 16 if the voltage of any of outputs 11-13 (i.e., V.sub.11, V.sub.12, and V.sub.13) drops below a threshold voltage (V.sub.th). In response to receipt of under-voltage signal 15, time delay circuit 16 waits for a selected interval, typically on the order of 250 milliseconds (ms), and then signals shutdown latch 18, which latches off power supply 10. The lengthy delay between the detection of the under-voltage condition and the shutdown of power supply 10 is required in order to avoid erroneously detecting a short circuit when an under-voltage condition occurs at power on.
Referring now to FIG. 1B, a timing diagram of the operation of the short circuit protection circuit is illustrated. As illustrated, the voltage of output 13 (V.sub.13) is initially constant at a desired output voltage such as 12 V DC. Then, at time t.sub.0, a short circuit fault occurs on output 13, which causes V.sub.13 to decline until V.sub.13 drops below V.sub.th at time t.sub.1. Accordingly, under-voltage detection circuit 14 transmits an under-voltage signal 15 to time delay 16, which triggers the shutdown of power supply 10 by shutdown latch 18 at time t.sub.2. Following shutdown of power supply 10, V.sub.13 is zero.
A principal drawback to the short circuit protection provided to power supply 10 is that the power supply components coupled to output 13, which has no current limit, are subjected to extreme stress during the long dim interval between times t.sub.1 and t.sub.2. Because of this stress, the components may fail, resulting in a catastrophic failure of power supply 10. It would therefore be useful and desirable to provide improved short circuit protection for a multiple output power supply.